The present invention relates to nuclear reactor plants and, more particularly, to isolation condensers for such plants. A major objective of the present invention is to provide an a simpler and more compact isolation condenser characterized by improved flow stability.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
Dual-phase reactors store heat generated by the core primarily in the form a phase conversion of a heat transfer medium from a liquid phase to a vapor phase. The vapor phase can used to physically transfer stored heat to a turbine and generator, which are driven to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling. Dual-phase reactors are contrasted with single-phase reactors, which store energy primarily in the form of elevated temperatures of a liquid heat-transfer medium. Pressurized water reactors (PWRs) are considered single-phase in that the reactor coolant is maintained in a liquid state, although heat from the pressurized water is used to boil a secondary coolant to drive a turbine. The primary example of a dual-phase reactor is a boiling-water reactor (BWR). The following discussion relating to BWRs is readily generalizable to other dual-phase reactors.
Modern BWRs provide for the removal of reactor decay heat from a reactor pressure vessel in the event the turbine becomes isolated from the reactor. During a turbine shutdown, a valve on the main steam line is closed preventing steam from reaching the turbine. Even after the reactor is shut down by fully inserting the control rods, decay heat continues to be generated for a period of days. This heat generates steam, which if left to accumulate in the reactor pressure vessel, could exceed the vessel's pressure-bearing specifications, potentially inducing a breach. An isolation condenser is one type of system designed to handle steam during turbine isolation to avoid excessive pressure accumulation.
A typical isolation condenser includes an upper distributor chamber and a lower collector chamber. The chambers are immersed in the water of a condenser pool. The chambers are coupled via an array of vertical tubes which extend therebetween and through intermediate pool water. During isolation, steam is conveyed to the distributor chamber. The steam is forced through the tubes, which through heat exchange with the condenser pool, condense the steam so that water flows into the collector chamber. A drain conduit coupled to the outlet chamber conveys the condensate to the reactor to replenish its coolant supply.
The performance of such a condenser can be impaired when the condenser pool has been heated to saturation. At that point, steam generated in the pool can insulate the heat-exchanger tubes, limiting further heat transfer and causing thermal cycling in the manifold. The thermal cycling can stress the condenser, impairing its structural integrity and inducing pool-side flow instability.
Other problems with such a conventional isolation condenser concern the amount of material required to ensure the distributor and collector can withstand the large pressure differentials that can develop between their interiors and the condenser pool. Pressure differentials of up to about 1250 pounds per square inch must be accommodated. The relatively flat boundaries, including the tube sheets, of the disk-shaped distributor and collector require considerable thickness to withstand this pressure. The thickness not only adds bulk and mass to the condenser, but subjects it to thermal stresses due to the larger thermal gradients that thicker material can sustain.
What is needed is a more compact, lightweight isolation condenser that is less subject to flow instability. In addition, the condenser should be economical to manufacture and maintain.